Energy monitoring and control systems are widely used in order to achieve sophisticated control of the loads in building electrical systems. The loads typically consist of a vast array of devices and sub-systems, the most prominent of which from an energy consumption standpoint are the components of the heating, cooling, and lighting systems. With the recent emphasis upon energy conservation and power utility savings, and the desire for increased effectiveness and reduced time in finding and correcting load problems, energy monitoring and control systems are rapidly becoming a required element of the electrical systems in large facilities such as factories, offices, schools, hospitals, and public buildings. Further, as such systems become less costly and easier to install, and/or energy conservation becomes increasingly important, their presence will become widespread in other applications such as private homes.
An important type of energy control system employs a control station that is coupled between the hot wire of the AC source and earth ground. A significant advantage of this type of energy control system is its relative ease of installation, since it requires no connection to the neutral wire of the AC source, which is often inaccessible in the wall-mount switchboxes of most buildings. Like most electrical circuits, the control station requires a power source to supply its operating power needs. However, as a load that is referenced to earth ground, the control station requires a power source that is also referenced to earth ground and that, like the control station, requires no connection to the neutral wire of the AC source.
Among existing approaches for providing a ground referenced power source without accessing the neutral wire, the most straightforward approach is to simply use a battery. Unfortunately, this has the disadvantage of considerable long-term monetary expense, as well as the major inconvenience associated with periodic failure and replacement of batteries. It is therefore highly desirable that the power source obtain its energy from the AC source.
One existing method is to use a current transformer connected in series with the hot line of the AC source. Another method, commonly found in dimmer switches for incandescent lights, employs an electronic switch such as a triac and uses the non-conducting portion at the start of each AC line cycle to obtain power for the control electronics. In addition to other shortcomings, both approaches have the serious disadvantage of being impractical for use in branch circuits that carry high levels of current, and are therefore ill-submitted for use in many applications.
Several types of off-the-shelf power converters, coupled between the hot wire of the AC source and earth ground, are capable of providing the power needed to operate a control station or other ground referenced load, but cannot do so without drawing an excessive amount of "ground leakage" current. The amount of leakage current that may be permissibly drawn from, or returned to, each ground is limited by at least two constraints. First regulatory and safety standards (such as those set forth by Underwriters Laboratories) typically limit the root-mean-square (RMS) value of the ground leakage current to less than about 5 milliamperes. Secondly ground-fault interrupters (GFIs), which are widely employed in residential applications, automatically "trip" and disconnect the AC source from the branch circuit if the RMS current flowing to earth ground attempts to exceed a value that is on the order of more than a few milliamperes.
It is thus apparent that a need exists for a power supply circuit that provides a reliable, cost-effective, and easily installed source of power for devices connected between the AC line and earth ground, and that draws low enough a ground leakage current to satisfy regulatory standards and to avoid nuisance tripping of ground-fault interrupters. Such a power supply would represent a significant advance over the prior art.